1084 ALANW.
W E B B AND
Lnovgcmic Chemistry
H. TRACY HALL
CONTRIL(UT1ON FROM ‘THS u E P A R T X E S T O F CHEAIISTKY, BRIGHAM YOUSGUSIVERSITY, PROVO,UTAH 84601
High-pressure Synthesis of Rare Earth Polysulfides1
Receiued M a y 30, 1969 New rare earth polysulfides with the tetragonal LaS, crystal structure were prepared for T m , Yb, and Lu. New polyniorplis of the rare earth polysulfides having the “cubic” LaSz structure were prepared for Gd, T b , Dy, Ho, Er, Tm, T b , Lu, and U The apparent pressure and temperature regions of stability as observed in the quenched products are delineated. The X-ray powder patterns are given for the new products.
Introduction The polysulfides of all but the heavicst rare earth elements are known. Biltz3”reported the synthesis of LaSz and CeS2 in 1911. Flahaut, et CL~.,~’’ found that the polysulfides of La, Ce, Pr, Nd, and Sm all had the same “cubic” s t r ~ c t u r e . These ~ compounds were prepared by treating the respective rare earth sesquisulfide with excess sulfur in a sealed tube. It was noted that, upon heating, SmS2 lost sulfur and became nonstoichiometric. This loss was reflected in a change of the apparent X-ray symmetry from “cubic” to tetragonal. Flahaut also reported the synthesis of the polysulfides of Eu, Gd, Dy, and Y , which were all nonstoichiometric with a lower limit of RS1.90and had the tetragonal structure. Unsuccessful attempts were also made to synthesize ErSz and YbSz. Ring and Tecotzky5 reported the synthesis of the polysulfides of Tb, Ho, and Er. The lower composition limit found was H O S ~ . An ~ ~ .attempt to synthesize TmSz mas unsuccessful. Upon progressing through the heavier rare earth elements, the temperature must be lowered to stay in the polysulfide stability zone and simultaneously the sulfur pressure must be increased. The application of high-pressure, high-temperature synthesis techniques would be expected to satisfy both of these requirements. M;e use the term “polysulfide” herein to describe compounds containing sulfur in excess of the rare earth’s trivalent stoichionietry requirements. Experimental Section The high pressures were obtained by use of a tetrahedral press designed by Ha11.6.1 The high temperatures were generated by an internal graphite resistance heater and a controlled ac power supply which provided a low-voltage, high-current source. (1) This research was supported by t h e National Science Foundation a n d t h e A r m y Research Office, D u r h a m , N. C., a n d is p a r t of a dissertation by t h e first a u t h o r . (2) Kow a n NAS-NIZC Fellow a t t h e Kava1 Research Laboratory, Washington, D. C. (3) (a) W. Biltz, Z . Elektiochem., 17, 668 (1911); (b) J. F l a h a u t , M . Guitt a r d , a n d M. Patrie, Bull. SOC.Chiin. F r a n c e , 1917 (1959). (4) CeSz was reported b y F. L. Carter, M e t . SOC.C o n f . , 16, 245 (1961). t o be of tetragonal or lower symmetry, a n d more recently J. A. hfarcon a n d 12. Pascard, Comfit. Rend., C266, 270 (1968), found CeSex, which is isotypic with CeS2, t o be monoclinic with a pseudocubic os pseudotetragonal X - r a y structure. Therefore, w e will use t h e terms “cubic” a n d “tetragonal” herein t o designate t h e a p p a r e n t X-ray diffraction symmetry, as observed in t h e powder patterns. ( 5 ) S. A. R i n g a n d M. Tecotzky, Inoi,s. Chem., 3, 182 (1964). (6) H. T. Hall, Reo. Sci. Insli., 29, 267 (1968). (7) H. T. Hall, ibid., 33, 1278 (1962).
This equipment allowed routine work to pressures of 70 kbars and temperatures of 2000”. Pressure and temperature calibrations were performed during the course of the work.8 The pressure calibration was based on the resistance transitions found in cerium, mercury, bismuth, thallium, ytterbium, and barium using the values given by Jeffrey, et al.Q The temperature calibrations were made a t four different working pressures spanning the working region using Pt-Pt-lOyo R h thermocouples. S o correction for pressure effects on the emf was made. Sulfur flowers of nominal 99.99% purity were used in the mixes. The rare earth metals (Nd, Gd, T b , Dy*, 110, Er, Tm*, Yb, Lu, and Y*) were all obtained in ingot form with a purity of 99.5‘;C or better, from either Alfa Inorganics (denoted by asterisks) or n‘uclear Corp. of America. The ingots were filed and sieved to - 100, then a stoichiometric mixture of a 1 :2 mole ratio of metal t o sulfur was weighed out. These were intimately mixed and stored in a desiccator for use as soon as possible. Oxygcn contamination was thus minimized but not necessarily eliminated. The sample charge was tamped into a small B N tube capped on both ends by B S disks. The B N tube was used to prevent formation of rare earth carbides. Contamination by the B N was assumed minimal since i t was observed that the B N sleeve was easily separated from the sample slug after a run. The B N capsule was placed in the graphite-tube resistance heater and the whole assembly was inserted into a pyrophyllite tetrahedron, using molybdenum strips for electrical conduction from the tetrahedron faces t o the graphite tube. The tetrahedron was then painted with a slurry of red iron oxide in methanol t o increase the surface friction, and the whole assembly dried a t 110’ for a t least 1hr. This baking increases the pressure obtained by a given ram load.1° The tetrahedron was placed in the press and the pressure slowly increased to a load of 400 psi oil pressure and then rapidly t o the pressure of interest, since experience has shown that the major part of the gasket formation occurs below about 250-300 psi. The power was increased to the desired wattage over an interval of about 15 sec. The power was held a t this value for a time of 3-8 min, inversely dependent upon the wattage used to offset partially expected kinetic effects, and then abruptly cut t o quench the sample. The samples were broken open immediately upon removal from the press and the product slug was extracted. X fragment of thc slug was then ground between two polished tungsten carbide flats, and the powder was placed in a 0.5-mm capillary for an X-ray diffraction powder pattern. The product was then identified from the X-ray film since the visual appearance of the samples varied little between runs. The systems of interest were studied over a pressure range of 14-70 kbars and a temperature range of 400-1900”. Temperatures above 1100O were not attempted at the lower pressures. ( 8 ) S. Eatough, Doctoral Dissertation, Rrigham Y o u n g Cniversity, Provo, Utah, 1968. (9) 12. N . Jeffrey, J. U . Barnett, H. H. VauHeet, a n d H . T. Hall, J , AbP1.
Phys., 37,3172 (1966). (10) I,. E. Millett, Doctoral Dissertation, Brigham Y o u n g University, l’rovo, U t a h , 1068.
Vol. 9, No. 5, Muy 1970
RAREEARTH POLYSULFIDES 1085
I n
3
150
A
n
A
-
0
u
T e t r a g o n a l
NdS2
“Cubic”
A
T b S Z
A
L!
A
A
ElOC 3
+ U
*
A
a W
AI.
“ C u b i c “
0
TbS2
_.-.-
a B W
“Cubic‘’
0
+
NdS2
50 N o
Reaction
NO
~ C
I
I
,
,
20 PRESSURE
,
.
I
,
,
,
I
b
”
“
“
P 20R E S
R e a c f i o n
’S U R“E ’
~
‘
’
60
40 (kilobars)
+
Figure 1.-Reaction product diagram for Nd 2s: X, no reaction; 0, “cubic” NdSa; A, P-Nd2S3. The scatter in the calibration data limited the definition of the run parameters to f 3 kbars for the pressure and about &7y0 for the temperature. From 21 to 53 runs of varying pressure and temperature conditions were made on each of the rare earthsulfur systems studied. All the rare earth-sulfur systems studied except those of Nd and Y exhibited one or two runs which gave complex X-ray patterns. For the systems with Er, Tm, Yb, and Lu the unknown material was found around 1000’ and 15 kbars, indicating possibly a new phase. For the remaining systems no pattern was noted in the positions of the unknown material in the P-T field. Figures 1-5 show reaction product diagrams of typical R 2s systems. They are not phase diagrams. They only show the product formed by quenching after application of high pressures and elevated temperatures. Figure 1 shows the results obtained with the Nd 2s system. “Cubic” NdSz was found a t nearly all pressures a t temperatures above 500-600’. At somewhat higher temperatures P-NdzSJ was also found a t the lower pressures, and only this product was obtained a t the higher pressures in the interval 900-1200”. This is consistent with the results of sealed-tube methods.lt At yet higher temperatures sealed tube methods gave Y-NdzSa. Above the P-Nd2S3 region in the high-pressure experiments only the “cubic” NdSz was found, possibly indicating the formation of a product which is not stable a t low pressures. No new compounds were found. The system Gd 2s gave results similar to those in Figure 2. No reaction was noted below 550-650”. Above this temperature and below about 20 kbars the previously known tetragonal GdSz phase was found. I n the region from 20 to 70 kbars and 690900’ material was found which appeared the same as the tetragonal GdSz physically but which exhibited a somewhat simpler X-ray diffraction pattern. It had the pattern of “cubic” La%. Both the tetragonal phase and the heretofore unknown “cubic”
+
+
+
(11) M. Picon and M. Patrie, Compt. Rend., 243, 1769 (1956).
60
4 0 ( kilobars)
I
+
Figure 2.-Reaction product diagram for T b 2s. This figure also applies to the systems Gd 2s and Dy 2s with a minimum pressure for the “cubic” phase of 20 and 25 kbars and no reaction below 550-650 and 500-600”, respectively (see text): X, no reaction; 0,“cubic” TbSt; A , tetragonal TbSz; ?,unknown phase.
+
+
GdSz were identified from their X-ray diffraction patterns by comparison with the published data of Flahaut, et aZ.4 Figure 2 shows the system T b 2s. It shows the low-temperature region of no reaction below 450-650”, the tetragonal phase region a t higher temperatures below 23 kbars and above about l O O O ” , and the region of the new “cubic” polymorph from 23 to 70 kbars and 550-900’. This system alone of these studied showed partial formation of the “cubic” form a t very high temperatures and the higher pressures. Further work indicated that this was not reproducible and thus possibly reaction while quenching through the “cubic” region led to these observations. Two other runs of mixed tetragonal and complex unknown forms were also observed in this region but not in a uniform manner. The reaction prochct diagram for Dy 2s is similar t o Figure 2. The no-reaction zone occurs below 500-600”. The new “cubic’ polymorph occurs between 25 and 70 kbars and between 500 and 900’. Elsewhere above the no-reaction zone tetragonal DySe occurs. The diagrams for the systems Ho 2S, E r 2S, T m 2S, and Yb 2 s are all similar t o Figure 3, which illustrates the Er 2s system. Ho 2s shows a no-reaction zone below 500-700” and a new “cubic” polymorph of HoSz was found between 33 and 70 kbars and in the temperature range 600-800”. At other regions above the no-reaction zone the previously known tetragonal polymorph was found. Also found a t three points above 1400” and at pressures of 51 and 68 kbars was a mixture of tetragonal HoSz and cubic Y-HOZSSwhich has recently been reI ported by Eatough, et aZ.12 Further investigation failed to reveal a definite phase region for the sesquisulfide. 2s. Figure 3 shows the reaction product diagram of Er The no-reaction zone is below 450-650’. The previously known
+
+
+
+
+
+
+
+
+
(12) N. Eatough, A. W. Webb, and H. T.Hal1,Inovg.Chem., 8,2069 (1989).
Inorganic Chemistry
ALANW, WEBBA N D H. TRACY HALL
lOSG
f
A
A
a
A
A
A
.I
A T e t r a g o n a i
E r S 2
T e t r a g o n o l
A
c c A
a
'a
A
- - - _- - Ax
No
R e a c t i o n
X
* X
L P R E S S U R E
00
+ +
Figure 3.-Reaction product diagram for E r 25. This figure also applies t o the systems of Ho, Tm, and Yb 2s. The minimum pressures for cubic formation are 33, 52, and 65 kbars, respectively. The no-reaction zones are below 500-700, 450600, and 400-450°, respectively. Symbols are the same as in Figure 2,
x
40 (kilobars)
product diagram for Y
Figure 5.-Reaction
A
A
U
T e t r a p o n o l
A
loo(
LuS2
A
A
W
K 3
F 4 (r
W
.a 1 w
+
+
5oc
+
+
'4
A
__----N o
+
I
1
/
I
20
I
I
PRESSURE
Figure 4.-Reaction
#
I
I
40 Lkilobors)
I
product diagram for Lu
.
1
60
+ 2s.
1
+ 2s.
tetragonal ErSz was found above this region below 41 kbars aud at all pressures above 750'. From 41 to 70 kbars in the temperature interval 500-750° the newly synthesized "cubic" ErSz was found. The no-reaction zone of the T m 2s system was found to be below 450-600". The previously unknown "cubic" TmSz was found in the pressure range 52-70 kbars and the temperature range 600-800". Tetragonal TmSz which was not previously known was found throughout the region studied above the noreaction region and outside the cubic region. The Yb 2s system was similar to the Ho 2s system. The no-reaction region was below 400-450". A new "cubic" YbSz was found in the temperature interval 400-600' at pressures of 65-70 kbars. A previously unknown tetragonal YbSz was found in the rest of the region above the no-reaction zone. A cubic yYbzSs was also found a t 1500-1900" and 35-60 kbars. As in the case of the -y-Ho& noted above, this was the first time this structure has been produced from the elements. These compounds have also been obtained in this structure from another sesquisulfide structure recently.12 There is a possibility that instead of the RzS3 composition the R3S4composition was formed in the above work, but these two compositions are isomorphous and, for the lighter rare earths, have the same lattice constant. The R3Sa compounds of holmium and ytterbium are also unknown previously. Figure 4 shows the Lu 2s system. The no-reaction zone is below 500-450". The previously unknown tetragonal LuSa was found at higher temperatures. An unidentified phase of complex X-ray structure was found above 1000" in the region 14-21 kbars. Several attempts were made to find a "cubic" polymorph of LuS?. A mixture of the tetragonal and "cubic" phases was finally formed in the vicinity of 650' and 95 kbars. Figure 5 illustrates the system Y 2s. The no-reaction zone is below 500-550". A new "cubic" US2 was found from 35 t o 70 kbars and from 500" to 1200". The known tetragonal YS2 was found elsewhere above the no-reaction zone. I n physical characteristics the rare earth polysulfides were all similar. They were all dark gray to black with a submetallic
+
150C
60
Vol. 9, No. 5, May 1970
DENSITIES OF
THE
RAREEARTH POLYSULFIDES 1087
TABLE I RAREEARTHPOLYSULFIDES
______ Density, g/cm3-----
-----Theoret--Compound
ICs2
RSi 7
Obsd
5.85 6.00 6.19 6.36 6.47 6.58 6.78 6.88 4.07
5.5 5.9 6.1 5.9 6.0 6.4 6.5 6.4 3.6
Tetragonal
YSz
6.11 6.27 6.47 6.63 6.75 6.87 7.06 7.17 4.35
L’dSz GdSz TbSz DySz HoS~ ErSz TmSz YbSz Lust US2
5.29 6.05 6.15 6.35 6.49 6.64 6.72 6.89 6.99 4.32
GdSi TbSz DySz HoSz ErSP TmSz YbSz LUSZ
Cubic
5.14 5.79 5.89 6.08 6.21 6.37 6.43 6.65 6.76 4.05
Figure 6.--The ErSez tetragonal subcell (from K. Wang, Doctoral Dissertation, University of Texas, Austin, Texas, 1967).
... 5.4 5.4 5.8 5.9 6.1 5.7
... ...
3.9
silvery sheen. They crushed readily to give a dull black powder which exhibited a variable brick red tinge. They all had rather high electrical resistance although no absolute measurements were made. An attempt was made to measure the densities of a number of the samples. Only 0.1-0.2 g of product was available for each determination so a precision of only =tlOyowas expected. Theoretical densities were calculated assuming both RSZ and RSl.7stoichiometries, since other work4sj indicates that there is a sulfur deficiency in the tetragonal polysulfides extending from SmS,.ooto HoSl 70. The density values are presented in Table I. N o chemical analyses were attemped due to the small sample size and due to the lack of a suitable technique for separating the unused reactants from the products. The rare earth polysulfides all reacted with aqueous solutions of HCl, HN03, and HzS04 to produce gas with dissolution of the compound. Aqueous KOH attacked the specimens only partially. Water caused a surface discoloration, and anisole, which was used as the displacement fluid in the density determinations, produced no change a t all. All chemical reactions were run for 4 days. All of the compounds made were reexamined by X-ray diffraction after 1 month t o determine their stability in the relatively dry air of the laboratory. A number of specimens were also checked after 3 months. No change in the X-ray diffraction films was noted.
X-Ray Diffraction Studies The “cubic” rare earth polysulfides have been dexed on the basis of the LaSz structure, which has tetragonal space group P4/nmm. It is related to ErSes (LaTea) tetragonal subcell (see Figure 6) by relationship
inthe the the
aLsSi = 2aErSez CLa&
=
CErSiei
The X-ray diffraction spectra were obtained by the Debye-Scherrer method with a 143.2-mm diameter camera. Nickel-filtered copper radiation was used. The d values were calculated using the values X(Ka)
1.5418 A and X(Ka1) 1.54050 A. The intensities were estimated visually. A preliminary indexing of the patterns was accomplished by comparison with previously published indexing of related systems. The lattice parameters were then calculated on an IBM 7040 computer using the least-squares program LSRSTR. l 3 None of the optional correction functions (absorption, eccentricity, and beam divergence) was used since this program was designed to work with single-crystal data. However, absorption did affect the d values of the low-angle reflections. The indexing of the rest of the observed reflections was accomplished by the use of the FORTRAN I V program POWDER.'^ This program calculates the expected powder pattern intensities for all d values of interest, using the lattice parameters, the L a d group, the necessary atomic scattering factor tables, the general positions, the special extinctions, and the atomic positions of the atoms in the asymmetric unit. For this work the atomic positions determined by Wang for ErSea15 were used. Wang found nearly the same parameters for both ErSez and NdTel.8. Owing to the similarity of the ionic radii of sulfur and selenium, it was assumed that these positions could be used for the polysulfide systems to calculate approximate reference powder patterns. These calculated patterns were then used to assign indices to the observed diffraction lines on the basis of the calculated and observed intensities. The preliminary indexing of both the tetragonal and the “cubic” R S aphases was accomplished by comparison with F l a h a ~ t . The ~ indexing of both structures was extended to include all observed lines by means of the calculated reference patterns. All the tetragonal compounds were then compared for consistency in indexing, and the same treatment was given the “cubic” compounds. The lattice parameters were then obtained from the full diffraction sets by a second application of the program LSRSTR. T h e errors given are *2u values. Limiting the fit to those lines with 28 values (13) M . H. Mueller, I,. H e a t o n , and K. T. Miller, A d a C ~ y s l . 13, , 828 (1960). (14) D. K. Smith, R e p o r t UCRL-7190, Lawrence Radiation L a b o r a t o r y . Livermore, Calif., 1963. (15) IC. Wang and H. Steinfink, Inorg. Chenz., 6, 1886 (1967).
1088 ALANW. WEBBAKD H. TRACY HALL
Inorganic Chemistry T A B L E 11 LATTICEPARAMETERS OF THE RAREEAKTHI’OLYSULFIDES
Mean atomic vol, A 3
Compound
RS2 Tetragonal Polymorphs GdS? TbSi DyS, HoS2 ErSn TmSz YbSz LUS2
7 . 7 9 6 i 0.008 7 . 7 5 4 i 0,009 7 . 6 9 6 i 0.004
7.649 i 0 . 0 0 3 7 . 6 3 6 i 0.006 7 . 6 1 0 z t 0.004 7.578i0.008 7 . 5 6 0 i 0.006 7 . 7 2 0 2 ~0 . 0 0 4
YS,
481.12 472.82 465.60 458.64 455.45 450.79
7.196 i 0 , 0 0 8
7 . 8 6 4 i0 . 0 0 9 7 . 8 6 1 % 0.004 7.839 i 0.003 7 . 8 1 1 1 0.006 7 . 7 8 4 + 0.005 7.767i0.008 7.751 i 0.006 7 . 8 4 6 % 0.004
20.05 19.70 19,40
19.11 18 98 18 78 18.59
446.03 44:3 00
18,46
467.61
19.48
514.11 489,68 482 81
21.42 20.40 20.12 1‘3.84
RS2 Cubic Polymorphs 8 . 0 1 1 i 0.003 7 . 8 8 2 i 0 003 7 . 8 4 5 i 0.006 7.809 i 0.004 7 . 7 8 4 i 0.004 7 . 7 4 5 i- 0.004 7 . 7 4 5 2 ~0.006 7 . 7 2 2 i 0.004 7.687 i0.012 7 . 7 8 7 i 0.006
NdSz GdSz TbSz UyS1
I-IOSA EL32 TmSp YbS? LuS* YS2
Ring a n d
Tecotzky (5) Present Work
7.6
1
75
,
‘.p
,Cf
..
Nd Pm Sm Eu Gd Tb YDy Ho Er T m Y b ILL ..r
,
1.10
0,90
1.00
IONIC
(A1
RnDlUS
Figure 7.-Lattice parameters of the “tetragonal” rare earth polysulfides. [Ionic radii used here and in Figures 8 and 9 are taken from 0. H. Templeton and C. H. Dauben, J . Am. Ckem. SOC., 76, 5237 (1954), with the exception of Y3+, which was taken from A. Iandelli in “Rare Earth Research,” E. V. Kleber, Ed., The Macmillan Co., New York, N. P., 1961, p 140.1
0 R i n g and
Present
Tecotzky Work
(5)
81
Lo
Ce
Pr
Nd
Pm
Sm E u Gd
Tb Y D y
7.6 I O N I C
Figure 8.-Lattice
Y
1.00
1.10
R A D I U S
Ho Er T m Y b L i .
.
0.00
..a
ta.)
parameters of the “cubic” rare earth polysulfides.
476,20
471.64 464.58 464,58 460.46 454,22 4T4.00
T A B L E 111 X-RAYI’OWDER PATTERNS O F THE T E T R A G O N A L RAREEARTHPOLYSULFIDE5 (A. %) _-TmS2----. __
Alrl 002 201 211 220 003 221 310 222 203 004 400 223 401 204 402 421 224 403 205 423 404 225,440 424 442 601 206 620 448,621 226 426, 622 603 444 623 604 641 426 624,227 605 643 407
Intens
19.65 1 9 . 36 1s).36 1919
18.93 1Li.25
I*US?-.-
dobsd
dcaled
dobed
doalcd
dohad
dcaloci
3.90 3.42
3 89 3.42
3.86 3.39
2.691
2.669
2.505 2.543 2.406 2.213 2.144 1,946 1.903 1.868
2,586 2.530
3.85 3.38 3 07 2.662 2.573 2.523 2.384 2.199 2.130 1.936
3.88 3.40
w
2 687 2.590 2.636 2.410 2.210 2.138 1.945 1.897 1.865 1.843 1.731 1.707 1.661 1.576 1.532 1.441 1,421 1,360 I . 346 1.281 1.271 1 251 1.228
3 88 3.41 3.11 2,679 2 589 2,533 2,396 2,205 2.138 1.942 1.895 1,862
m
1.203
vw vw
1 . 195
vvs
1,148
vw
1.139 1.106 1.090
1.709 1.662 1,577 1.534 1.441 1.423 1.360 1,346 1.281 1.271 1 252 1,228 1.203 1.192 1,169 I 119 1.139 1,107 1.092
1,045 1.032
1,046 1,032
0.983 0 978
I
7.2
vvs vvw
s w 1%’
vvw vvs
vvs
vw s vw
vvw VIV
m s
vw vw 5.2
vs
vw
m vw w IV
vvw
vw
1 169
1.848 1.733
vvw
w w vvw
vvw
m vvw
,0 , 9 8 8
0.978 I0 . 9 6 0 0 960 + 2 7 other lines
2.208
2 134 1.942 1.890 1.863
1.841 1,726 1.700 1.654
1.574 1,527 1.438 1.418 1.355
1.346 1,276 1.266 1 247 1 224 1 198 1,187
1.166 1,144 1.134 1.102 1.087 1.056 1.031 1.029 1,020 0.980
1.728 1.703 1,656 1,572 1.629 1.437 1 418 1,356 1.342 1.277 1.266 1,247 1.225 1.198 I . 187 1.166 1.146 1.135 1.103 1.087 1.050
3.10
2.673 2.584 2.527 2.391 2.200 2.13:+
1.938 1.890 1 . 8 5 5 1.858 1.833 1.836 1 . 7 2 5 1 724 1 . 6 9 5 1.699 1 . 6 4 8 1.652 1 . 5 6 8 1.569 1.523 1.525 1.434 1.434 1.412 1.415 1.352 1.353 1 . 3 3 5 1.3:1‘3 1.272 1.274 1.885
1.261
1.242 1.222 1,1!9,5 1 183 1.163 1.141 1 131 1.099
1.263 1.214
1.222 1.195
1.184 l.l6:3 1.112
1.041 1 029
1.133 1.100 1.084 1.085 1.054 1.056 1.038 1.039 1 027 1 . 0 2 6
1.022 0.080 0,974 0.957
1 . 0 1 7 1.020 0.978 0.978 0 971 0 . 9 7 1 0 . 9 5 6 0.955
0 974 0.958 + 2 2 other lines
1 2 5 o t h e r lines
from 90 to 180”decreases the error by a factor of about 2//3 in most cases. This error is closer to that found by other workers for members of this series. The lattice parameters are given in Table I1 and compared with previous work in Figures 7 and 8. The powder patterns of the tetragonal polysulfides of Tm, Yb. and I,u are given in Table 111, and the powder patterns of
RAKEE A R T H POLYSULFIDES
Vol. 9, No. 5, Muy 1970 TABLE IV
X - R A Y POWUEIi PATTERNS OF THE “CUBIC” ILARE EARTH POLYSULFIDES
-
___
I
Ho S z
Tb S
Gd S
h kl
d obr
-- __ -__ __ __ 002 201
-
211 I
W
vvs VVW
vvw VVW
__
vs
003, 221 310 311
vs v VW vvw
220
__
222 203 321 400
-
401, 223 303, 411 331 402
-
421 332 422 005, 403 105, 413 115, 333
-
205, 423 440 225, 441 305, 433 006, 442 601 611, 532
--
62 0 105, 621, 443 533 622 603, 425 631 444 007, 623 604 207, 641 426 605, 643 615, 732
W
VVW VV s
vvs
d,,lC
d Ob3
3.91 3.50 3.30
3.94 3.53
3.93 3.49
2.771 2.622 -
3.21
-
-
2.270 2.183
VVW
-
S
1.962
VVW W
1.911
vvw W W V W S
vw VVW W VVW V W VVW
vvs W
vw VVW W V vw VVW W W W VVW W W V W VVW VVW VVW
VVW W
W
vvw
-
-
-
1.756
-
1.715
-
1.575
-
-
-
1.463 1.390 1.373
-
1.314
-
1.249 1.231
-
1.055 1.00E
-
-
2.787
-
2.627 2.486 2.370
__ -
2.940 2.760
2.616 -
-
-
2.275 2.263 2.186 2.178 2.101 1.971 1.956 1.929 1.912 1.902 1.853 1.847 1.803 1.762 1.754 1.738 1.720 1.710 1.676 1.605 1.600 1.576 1.572 1.546 1.538 1.513
-
-
-
-
-
-
i C * l C
3.90 3.49
3.20
2.752 2.603 -
-
2 774
-
2.615 2.479 2.363
-
-
-
ohr
3.18 2.761 2.603 2.466 2.351
-
3.89 3.46
2.738 2.587
-
2.388 2.240 2.162
3.89 3.48
3.18 2.752 -
2.595 2.457 2.343
-
-
-
2.254 2.166 2.084 1.952
-
1.939
1.903 1.849 1.798 1.754
1.890
1.894 1.841 1.789 1.746
1.886
1.739 -
1.888 1.835 1.783 1.741
1.712 1.671 1.601 1.569 1.539 1.509
1.700
1.704 1.662 1.594 1.562 1.531 1.501
1.695
1.699 1.657 1.589 1.557 1.527 1.495
1.451 1.450 1.378 1.380 1.364 1.359 1.337 1.301 1.302 1 , 2 8 0 1.284 1.267
1.446 1.372 1.360
-
-
-
-
1.746
-
-
1.563
-
-
-
1.236 1.235 1.218 1.220 1.189 1.178 1.177 1.166 1.164 1.151 1.126 1.114 1 . 1 1 6 1.081 1.070 1.073 1.047 1.044 1.001 1.000 0.992
-
-
-
-
1.584 1.556 1.523
-
-
-
-
-
-
1.297 1.275 1.256 1.237 1.228 1.214
-
1.172 1.163
-
1.110
-
1.068 1.044 0,997 0.986
-
-
Discussion Stoichiometric ‘(cubic” rare earth polysulfides were previously known for the elements lanthanum through samarium, and sulfur-deficient tetragonal rare earth polysulfides were knowii for the elements samarium through erbium4r6 The degree of sulfur deficiency increases through the series, with SmS1.90 at the first and DyS1.89 or HoS1.72 later in the series. Since no chemical analyses were made, only an estimate of the probable composition of the new tetragonal polymorphs can be made. The theoretical densities based on the RS2composition show the tetragonal forms to be denser than the cubic polymorph. Since the “cubic” polymorph is the high-pressure form, i t would be expected to be the denser of the two forms. Thus, if we assume that the “cubic” form has the RS2 stoichiometry and that the theoretical density of the tetragonal form is equal to that of the “cubic” form, we can calculate an upper limit for the composition of the tetrag-
2.582 -
3.87 3.46
-
3.16
-
2.738
-
2.582 2.446 2.332
3.88 3.44 3.33 3.07 2.791 2.718 2.687 2.576
-
-
dCelC
3.87 3.46
-
3.16
-
2.738
-
2.582 2.444 2.330
-
3.85 3.43 3.30 3.13
3.86 3.45
-
3.15
2.714 2.677 2.575 2.405
2.730
-
-
2.574 2.436 2.323
-
2.236 2.229 2.236 2.223 2.229 2.148 2.150 2.148 2.140 2.142 2.070 2.065 2.070 2.064 1.936 1.934 1.936 1.931 1.931 1.914 1.879 1.878 1.872 1.878 1.864 1.873 1.826 1.817 1.820 1.814 1.816 1.774 1.775 1.777 1.772 1.731 1.732 1.732 1.732 1.725 1.727
-
-
-
-
-
-
3.91 3.41 3.28 3.15 2.993 2.740
3.90 3.49
-
2.757
2.584 2.479 2.393 2.301 2.242 2.164 2.070 1.941
2.599 2.466 2.351
1.887
1.891 1.833 1.784 1.743
-
1.745
-
-
-
-
1.685
1.690 1.849 1.581 1.549 1.519 1.488
1.684 1.664 1.592 1.547
1.690 1.651 1.581 1.549 1.519 1.491
1.681 1.685 1.648 1.646 1.576 1.576 1.540 1.544 1.507 1.514 1.486
1.693
1.438 1.369 1.348 1.328 1.291 1.273 1.254
1.434 1.361 1.345 1.323 1.284
1.434 1.365 1.344 1.324 1.287 1.269 1.250
1.230 1.225 1.207 1.210 1.185 1.181 1.169 1.168 1.155 1.155 1.138 1.142 1.115 1.118 1.100 1.106 1.074 1.074 1.061 1.064 1.036 1.035 0.988 0.992
1.223 1.203
1.221 1.206 1.178 1.165 1.151 1.139 1.115 1,103 1.071 1.061 1,032 0.989
1.449 1.375 1.360 1.333 1.298 1.279 1.258 1.239 1.235 1.217
-
1.581 1.548 1.519
-
1.445 1.440 1.376 1.367 1.355 1.352 1.333 1.297 1.290 1.280 1.274 1.263 1.250 1.234 1.231 1.223 1.216 1.208 1.185 1.173 1.169 1.160 1.157 1.148 1.140 1.124 1.119 1.112 1.105 1.079 1.076 1.069 1.062 1.040 1.039 0.997 0.992 0.989 0.983
-
-
+ l other line +I o t h e r lines + 3 o t h e r l i n e s +4 other lines
the (‘cubic’’polysulfides of Gd, Tb, Dy, Ho, Er, Tm, Yb, and Y are given in Table IV.
3.87 3.44 3.25 3.14 2.904 2.728
2.393 2.247 2.229 2.159 2.152 2.080 2.073 1.946 1.930
2.265 2.251 2.176 2.170 2.095 1.961 1.947
-
-
-
S2
d,,,, de,,, d d c a i c d obs d c a i c dohr ----___ __ - - ~
3.90 3.48
-
-
Tm
1 C 8 1 ,
~
3.92 3.51
1.460 1.457 1.387 1.387 1.369 1.366 1.344 1.308 1.308 1.287 1.290 1.269 1.273 1.246 1.246 1.240 1.240 1.231 1.225 1.225 1.199 1.195 1 , 1 8 8 1.183 1.183 1.175 1.171 1.169 1.159 1.156 1.157 1.132 1.138 1.126 1.119 1.121 1.087 1.093 1.083 1.07E 1.078 1.048 1.050 1.053 1.009 1.003 1.004 0.995 0.996
1.464 1.393 1.372 1.352 1.3 14 1.296 1.279
E r S:,
__ __ __ __ __
-
1.498 1.481 1.437 1.365 1.350
-
1.438 1.369 1.348 1.327 1.291 1.273 1.256
-
1.289 1.274
-
-
-
1.225 1.210 1.180 1.168 1.155 1,142 1.118 l.lOE 1.074 1.064 1.03: 0.992 0.984
~5 o l h e r line!
-
-
-
-
--
--
-
1.164 1.152
-
1.115 1.096
-
1.058 1.033 0.987
-
-
-
-
-
-
1.591 1.558 1.525
-
-
1.175 1.165 1.145
-
1.112 1.064 1.046 0,999 0,989
-
3.18
-
-
2.251 2.163 2.084 1.949
-
-
1.701 1.658 1.592 1.559 1.529 1.496
-
1.448 1.378 1.357 1.337 1.300 1.282 1.265
-
1.233 1.218 1.186 1.175 1.162 1.150 1.123 1.114 1.078 1.071 1.042 0.998 0.99c
c4 o t h e r l i n e s -8 o t h e r l i n e s c9 other line!
onal polymorph.
Then we find the compositions
TmS1.7R,YbS1.77, and LuS1.83. The density of the tetragonal form is expected to be less than that of the “cubicJJpolymorph and there is a possibility that the heavier “cubic” rare earth polysulfides are sulfur deficient. Both of these factors would tend t o lower even further the sulfur content of the tetragonal polymorphs from the above values. This is in line with the known compositions of the lighter members. T h e appearance of the “cubic” phase, definitely of the same X-ray powder diffraction pattern as the re2s from highported “cubic” La&, in the system Gd pressure synthesis might be due to either or both of two reasons. The first is the relative compressibility of the sulfur in its negative ionic state (whether as S2- or S22- ions) with respect to the metal ion. The metal ion has been found to be about one-fourth as compressible as the sulfur;le thus the metal to anion radius ratio increases with pressure. However, the metal to anion radius ratio decreases for a given pressure upon going through the lanthanide elements from
+
(16) C. P. Kempter, P h y s . Slalus Solidi, 8, 161 (1965).
l n o r g m i c Chemistry
70
W40Lz 3 v)
(0 W
30-
E
a
-
zo -
I
t
Gd
Tb
Ho
YDy I
Er ,
,
Tm ,
IONIC
Figure 9.--The
RADIUS
Yb ,
Lu I
,
085
0.90-
(4)
minimum formation pressure for RSn “cubic” polymorphs.
lanthanum to lutetium. Thus, the effect of pressure is to cause a given system to mimic the system containing a lighter rare earth element a t a lower pressure. Formation of the “cubic” polymorph would therefore indicate mimicry of samarium or a lighter element, and the minimum pressure necessary for formation of the “cubic” form would increase as a function of the difference between the atomic number of the element of interest and that of samarium or as a function of the difference in the respective ionic radii. The observed miminum pressure of formation for the “cubic” phase is shown as a function of the ionic radius in Figure 9. The second possible reason is a return to the higher sulfur content seen with the lighter rare earth polysulfides. WangI7 found from his single-crystal studies on NdTel.8 that the vacancies occurred in the basal plane, ;.e., in the dense-packed layer of anions (see Figure 6). He concluded that the polpselenides were (17) R. W a n g , Doctoral Dissertation, University of Texas. Austin, Texas, 1967.
similarly deficient, and thus it is logical to suppose that the vacancies in the polysulfides also occur in the basal plane. This is also suggested by the data on Sins2, n-hich Flahaut, et have found to be pseudocubic (a = 7.97 A) but which becomes tetragonal upon heating with a loss of sulfur (a = 7.89, c = 7.97 8).This shrinkage of the a lattice parameter with no change in the c parameter supports the view that the anion vacancies occur in the basal plane. The filling of at least part of these anion vacancies would cause a return to the pseudocubic X-ray powder symmetry, and from the work of Ring and Tecotzkyj it would be expected that higher sulfur pressures would be required for this. Thus, the increase in a minimum pressure of formation would be related to the minimum sulfur pressure necessary for synthesis. Since the “cubic” samarium polysulfide is also nonstoichiometric (SmSI.94), and the density data indicate that the ‘‘cubic” polymorphs of the heavier rare earth elements are probably sulfur deficient, it seems likely that both of the above reasons are contributing to the formation of the high-pressure “cubic’! phase. I t will be noted in Figure 9 that a smooth curve is observed to fit the values of minimum formation pressure for the “cubic” form us. rare earth ionic radius fairly well with the exception of the value for yttrium. A better fit is obtained if yttrium is assigned an ionic radius of about 0.890A. Iandelli18 gave it the value of 0.9108,and Ring5 similarly placed Y between T b and Dy. Piconlg placed it very close to Dy (0.90 A) and EatoughlO placed i t a t 0.923 A, the same as Tb. These values would predict the formation of the “cubic” phase a t about 10 kbars lower than observed. Two effects are probably a t work here. The first is the somewhat variable ionic radius of yttrium j second, yttrium is not one of the lanthanide elements and thus is not expected to act completely like one. Acknowledgments.--\I:e thank James H. Hoen, Korman L. Eatough, John F. Cannon, Karl Miller, M , D . Horton, and Fred D. Childs for their assistance in various phases of this work. lye thank Leo Alerrill and the Brigham Young University Computer Research Center for help with the computer programming. (18) A. Iandelli in “ R a r e E a r t h Research,” E. V. Kleber, E d . , T h e Macmillan Co., New York, K. Y . , 1961, pp 146-151. (19) hI. Picon, L. Domange, J . F l a h a u t , RI. G u i t t a r d , a n d &I. Patrie, Bull. 5‘06. Chin?. F r a m e , 27, 221 (1Y60).
